408 
PALEONTOLOGY OF NEW-YORK. 
of concentric striae or lines of growth are sometimes preserved. The 
radiating striae on the upper lateral portions of the shell are curved 
towards the dorsal margin, gradually increasing in the curvature till 
(as shown in the cast of the dorsal valve) the direction becomes re¬ 
versed. 
The interior of the dorsal valve shows a very subordinate cardinal process 
and two strong lamellae or brachial processes, which are joined to the 
shell by a ridge from the base on either side, and a well-defined 
median septum extending from the base of the cardinal process : the 
vascular impressions are not distinctly seen. The interior of the ventral 
valve is marked by strong muscular imprints which occupy a large 
cordiform space, reaching more than half and often two-thirds of the 
length of the shell from beak to base : the central portions, or the 
imprints of the adductor muscles, are often well defined. Surfaces of 
muscular imprint marked by large radiating grooves and ridges in 
old shells; while in the young shells, these markings are much less 
conspicuous or altogether wanting, and the outline limit of the mu¬ 
scular impression itself is but faintly defined along its lower margin. 
This is the largest Orthis known to me in our rocks, and is quite equal in size 
to any of the Carboniferous species which have fallen under my observation. It is 
readily distinguished by its large and strongly marked muscular impression, which, 
however, presents a considerable variation in the degree of development dependent 
on the different stages of growth, and in some measure apparently upon the nature 
of the sediment in which it lived. 
PLATE LXXXIX. 
Fig. 1 a. A dorsal valve which retains a small portion of the shell. The direction of the striae 
upon the hinge-margin is very well shown. 
Fig. 1 b. Profile view of the same. 
Fig. 2 a. A similar cast, showing the vascular impressions upon the surface of the more 
prominent portions, and likewise towards the anterior margin. 
Fm. 2 b. Profile of the same from the cardinal side. 
O 
Fig. 2 c. The interior of the upper part of the dorsal valve and brachial processes, as shown 
in a mould from the preceding figure. 
Fig. 4. The cast of a shell of medium size, preserving the muscular impression in its usual 
symmetrical form. 
Fig. 3. A very old specimen, in which the muscular impression is extremely elongated. 
